There have been developed internal combustion engines which are capable of mixing and combusting liquid fuel such as light oil, gasoline, etc., and compressed natural gas (hereinafter also referred to as CNG). For example, in Patent Document 1, there is disclosed an auxiliary fuel ignition type gas engine which uses light oil as auxiliary fuel. In this auxiliary fuel ignition type gas engine, mixed combustion of light oil and natural gas is carried out using the light oil as an ignition source. In addition, in Patent Document 1, there is described a technique in which at the time of light load, only light oil is supplied to the internal combustion engine, and at the time of medium load and high load, light oil and natural gas are supplied to the internal combustion engine.
In Patent Document 2, there is disclosed a fuel injection control device which controls fuel injection into cylinders of an internal combustion engine. This fuel injection control device is provided with a reducing agent supply unit that supplies a reducing agent to an exhaust gas purification device. This reducing agent supply unit supplies the reducing agent to the exhaust gas purification device by performing, after main fuel injection, post injection as well as before-post injection small amount injection (pre injection) which is carried out before the post injection and which is smaller in the amount of injection than the post injection, by means of fuel injectors which serve to inject fuel into the cylinders, respectively.
In Patent Document 3, there is disclosed that two or more of an oxygen concentration, a CO concentration and an HC concentration, of an exhaust gas are used as control parameters at the time when a reducing agent is supplied into an exhaust gas flowing into an exhaust gas purification device.